This invention relates to corneal contact lenses and more particularly to monovision type corneal contact lenses.
Presbyopia is a condition of advancing age caused by a hardening of the natural crystalline lenses of the eyes. The hardening results in a resistance of the lenses to changes in surface curvature, thereby usually rendering effective near vision impossible without corrective lenses. Conventional contact lens correction for presbyopia includes contact lens equivalents of traditional spectacle bifocals: each contact lens is divided into a D-shaped upper area adapted for distance vision and a D-shaped lower area adapted for near vision. Other conventional contact lens corrections for presbyopia may take the form of annular, aspheric or diffractive lenses.
Traditional bifocal eyeglasses are effective for everyday use because the eyeglasses sit in a fixed position on the wearer's nose. To change from distant vision to near vision or vice versa, the wearer merely shifts which portions of the lenses through which the eyes are looking. The shift may be effectuated either by a rotation of the eyes or by a tilting of the head, or both.
When the same geometric configuration is used in contact lenses, the bifocal correction is not entirely effective because a contact lens slides relative to an eye surface. Upon a rotation of the eye in its socket, the lens may encounter an eyelid, which moves the lens relative to the eye and particularly relative to the pupil.
Other configurations of the correction zones in bifocal contact lenses also suffer from a shifting of the contact lenses relative to the respective eye surfaces. In all conventional bifocal contact lenses, shifting causes blurring and a concomitant success rate of only about 25%. The highest success rate attainable for bifocal or multifocal contact lenses is at most 35%.
The low success rate of conventional multifocal corneal contact lenses also arises from changes in pupil size. Because the diameter of the pupil changes with variations in ambient lighting conditions, and because of the shifting of the lens with respect to the corneal surface, the relative amounts of light falling on the retina through the different correction zones of a multifocal lens differ under different light conditions. The differences aggravate the problems exhibited by multifocal contact lenses.
Generally, multifocal contact lenses (usually either bifocal or trifocal lenses) not of a segmented configuration are of a concentric configuration. In a conventional bifocal contact lens of the concentric type, a first, centrally located, circular correction zone constitutes a distant vision correction zone and is defined by a relatively long focal length, while a second annular correction zone surrounding the first zone is a near vision correction zone and has a relatively short focal length.
A trifocal contact lens is provided with a third correction zone having a focal length with a value lying between the distant vision focal length and the near vision focal length. The third correction zone may be termed a middle or an intermediate distance correction zone.
A common problem with conventional multifocal contact lenses is that a portion of the actual perceived image is always out of focus. Such a situation occurs during night driving when the pupil is open to a maximum or near maximum diameter and light enters the eye through both the central correction zone and the annular correction zone of a concentric contact lens. This results in a large blur circle and concomitant difficulties for the driver. Theoretically, the user's brain is capable of ignoring the blurry portion of the image in favor of the focused portion However, this rejecting of the out-of-focus portion does not always occur.
An alternative type of contact lens for correcting presbyopia is the monovision lens. A monovision lens is designed to focus only near objects or far objects on the retina. Thus, a patient provided with a distant vision contact lens for one eye (usually the dominant eye) and a near vision contact lens for the other eye uses the one eye to distinguish objects at a distance (for example, while driving) and the other eye for near vision (for example, reading). Although patients using monovision contact lenses are not seeing stereoscopically, typically visual tasks can be better performed with monovision lenses than with bifocal contact lenses. In addition, monovision lenses have a 75% success rate, much higher than the 25-35% success rate of conventional bifocal contact lenses. Despite the superior performance of monovision type contact lenses, they suffer from the disadvantage that the consuming public finds the loss of stereoscopic vision to be undesirable.